This invention relates to flat panel displays having a baseplate used to generate an image and a faceplate through which the image is adapted to be viewed, and, more particularly, to a method of evacuating and sealing the space between the faceplate and baseplate, as well as flat panel displays and components fabricated using such method.
Flat panel displays are commonly used for a variety of purposes, such as for notebook computer displays, display panels for electronic instruments and devices, and portable televisions and camcorders, to name a few. One commonly used flat panel display is a liquid crystal display in which an image is generated by applying signals to a baseplate to selectively modulate the transmissivity of a liquid crystal filling the space between the baseplate and a transparent faceplate. The transmissivity of the liquid crystal is modulated on a pixel-by-pixel basis to generate a monochromatic or color image that is visible through the faceplate.
Another type of flat panel display that has been proposed for use in a wide variety of applications are field emission displays. Field emissions displays employ a baseplate having a substrate containing an array of emitters or emitter sets (a group of emitters connected to each other) and an extraction grid surrounding each of the emitters. The emitters are generally biased between xe2x88x9230 and 0 volts, while the extraction grids are generally biased between 30 and 100 volts. When the voltage of an extraction grid is more positive by a sufficient voltage (e.g., 60 volts or more), electrons are extracted from the emitter.
The field emission display also includes a faceplate that is spaced apart from and parallel to the baseplate, thereby forming a space between the baseplate and faceplate. The surface of the faceplate facing the baseplate is coated with a layer of transparent conductive material, such as indium tin oxide (xe2x80x9cITOxe2x80x9d). Finally, the transparent conductive layer is coated with a layer of a cathodoluminescent material. The cathodoluminescent material may be applied to the transparent conductive layer either uniformly or in a pattern corresponding to a desired image. Different cathodoluminescent materials also may be applied in different locations to create color images or images of multiple colors.
In operation, a large positive voltage, on the order of 1000 volts, is applied to the conductive layer coating the faceplate to draw the electrons emitted by the emitters to the conductive material. The electrons traveling to the faceplate strike the cathodoluminescent material, thereby giving up their energy thus causing the cathodoluminescent material to be illuminated and portray an image which may be viewed through the transparent faceplate. The space between the baseplate and the faceplate of field emission displays must be evacuated and remain evacuated after prolonged periods of use. Should the space between the baseplate and faceplate not be adequately evacuated, residual gases in the space, when energized by the extracted electrons, can start arcing or even glow discharge thereby seriously limiting the operating and useful life of field emission displays. The emitters must also be precisely aligned to predetermined areas of the cathodoluminescent material. A gettering agent, such as ST-122 gettering agent sold by SAES, is placed in the space between the baseplate and the faceplate to maintain the vacuum environment at times of use and storage.
As illustrated in FIGS. 1 and 2, a faceplate 10 is generally glass or glass/ceramic, and generally a sealing material frit, such as glass or metal, forms a bead 12 extending around the perimeter of the faceplate 10 near its periphery 14. To assemble a field emission display, a field emission display baseplate 20 (FIG. 2) is placed over the faceplate 10 against an interface surface 24 formed along the upper surface of the frit bead 12. The plates 10, 20 are heated above a temperature at which the frit bead 12 will flow, either under the weight of one of the plates or with the application of a compressive force upon the plates 10, 20. The frit bead 12 is thus compressed, thereby causing the frit bead 12 to flow or extrude and adhere to the plates 10, 20. After the plates 10, 20 cool, the frit bead 12 forms a seal between the plates 10, 20.
The above-described procedure would be entirely satisfactory if it was not necessary for the space between the plates 10, 20 to be evacuated. However, as mentioned previously, the space between the plates 10, 20 must be substantially evacuated. There are two primary approaches to evacuating the space between the plates 10, 20. In one of these approaches, a tube or other conduit (not shown) is embedded in the frit bead 12. A hole could also be formed in one of the plates, such as by drilling. An evacuation tube would then be fritted to the plate overlying the hole during thermal processing of the plates 10, 20, to form a rigid, airtight seal between the plates 10, 12. The space between the plates 10, 20 is evacuated by means of vacuum pumping through the tube or conduit, and the tube or conduit is thermally or resistively heated to collapse and seal itself. This approach has several disadvantages, including the expense of creating and then sealing the tube or conduit, and the potential for subsequent leakage or possible breakage. Also, this approach requires space for the tube or conduit, which may be difficult in some applications, such as in laptops, where packaging space is limited.
Another conventional approach to evacuating and sealing the space between the plates 10, 20 is illustrated in FIG. 3 and described in U.S. Pat. Nos. 5,697,825 and 5,788,551 to Dynka et al., which are incorporated herein by reference. In this approach, frit protrusions 28 are formed at spaced apart locations along the frit bead 12, such as at the comers of the face plate 10. When the baseplate 20 is placed on the faceplate 10, the protrusions 28 position the baseplate 20 above the interface surface 24 of the frit bead 12. As a result, gaps 30 are formed between the baseplate 20 and the interface surface 24 of the frit bead 12 and provide a flow path for evacuating the space defined by the plates 10, 20 and the frit bead 12.
After the baseplate 20 is placed on the protrusions 28, the plates 10, 20 are placed in an evacuation oven (not shown). The evacuation oven heats the plates 10, 20 in an environment of substantially zero pressure. As the pressure in the evacuation oven is reduced, residual gases, comprised mainly of air, are drawn through the gaps 30 from the space between the plates 10, 20. After the space between the plates 10, 20 has been substantially evacuated, the plates 10, 20 are heated to a temperature at which the frit bead 12 will flow. A compressive force applied to the plates 10, 20 causes the protrusions 28 to totally collapse into the frit bead 12 and the frit bead 12 to partially compress. The frit bead 12 then bonds to the faceplate 20 so that a hermetic seal is formed between the plates 10, 20. The plates are allowed to cool before removing the vacuum from the evacuation oven and exposing the plates 10, 20 to atmospheric pressure. The result of this procedure is a hermetically sealed, evacuated space between the plates 10, 20.
Although the above-described procedure has several advantages over the use of a tube or conduit to evacuate a space, it nevertheless can be improved. First, the need to form protrusions 28 on the frit bead 12 adds to the time and expense of manufacturing field emission displays. Second, it is difficult to ensure that all of the protrusions 28 are of exactly the same size. Yet it is possible for sized protrusions 28 to allow the frit bead 12 to compress unevenly. Although spacers (not shown) are generally used to space the faceplate 10 a fixed distance from the baseplate 20, it is nevertheless undesirable for the frit bead 12 to compress unevenly. As a result of these and other problems, the use of protrusions 28 may be less than ideal for the tight tolerances between the baseplate 10 and the faceplate 20.
Another approach is described in U.S. Pat. No. 5,827,102 entitled xe2x80x9cLow Temperature Method For Evacuating And Sealing Field Emission Displays,xe2x80x9d incorporated herein by reference. In this approach, a low melting temperature material such as indium, is applied as a seal material. A flow path for evacuation is provided by non-conformance of the seal material with opposing surfaces.
The inventions defined by the respective claims are directed to methods for evacuating and sealing a flat panel display of the type having first and second planar plates. A frit bead of substantially uniform thickness is initially formed along the periphery of one of the plates so that an interface surface facing away from the plate is substantially flat. The plate and frit are then heated to a temperature that pre-glazes the frit bead. As a result, a plurality of surface irregularities are formed on the interface surface of the frit bead. The other plate is then placed on the interface surface of the frit bead so that a space is at least partially defined by the platen and the frit bead. The environment surrounding the plates is then evacuated. However, prior to placing the plates together, a getter may be placed between the plates. The surface irregularities on the interface surface of the frit bead provide flow paths between the frit bead and the other plate which allow gasses to flow from the space between the plates. As a result, the space between the plates equalize to the vacuum surrounding the plates. After the space between the plates has been evacuated, the plates and frit bead are heated to a temperature above the temperature at which the frit bead at least partially flows. A force is then applied to the first and second plates to move the platen toward each other. The applied force, along with the elevated temperature, causes at least partial compression of the frit bead. Finally, the plates are allowed to cool below the temperature at which the frit bead at least partially flows. This results in a vacuum in the space between the plates and a hermetic seal being formed by the frit bead. The vacuum is then maintained by the getter, which is activated during formation of the seal. Furthermore, this result has been accomplished without the addition to the field emission display of any structural component.
The frit bead preferably comprises either glass or metal particles mixed with a binder, although other compositions may be used. The plate on which the frit bead is formed is preferably heated to a temperature in the range of 250-350 degrees centigrade for a period of approximately 1 hour to pre-glaze the frit to form the surface irregularities. Pre-glazing the frit also reduces the binding agents, such as organic solvents and water. In contrast, the plates are preferably heated to a temperature in the range of 350-500 degrees centigrade for a period of approximately 1 hour to heat the plates to a temperature above the temperature at which the frit bead at least partially flows. The plates are preferably allowed to cool in two stages. In the first stage, the plates are preferably allowed to cool to a temperature in the range of 350-425 degrees centigrade, and they remain in that range for a period of time of approximately 1-2 hours to further activate nonevaporative getter material. In the second stage, the plates are preferably allowed to cool to ambient temperature over a period of at least 3 hours. The step of applying a compressive force upon the plates may be accomplished by a variety of techniques, such as by stacking the plates and placing a weight on them, or by applying a compressive clamping force upon the plates. Prior to joining and sealing the two panels to each other, a plurality of spacers may be placed between the plates. The spacers should have a spacing height that is less than the distance between the height of the frit bead so that the frit bead compresses to the level of the spacers.